= Ion-density convention in the Debye screening length
The <Debye length> depends on the density of every mobile ionic species, weighted by squared charge. For dilute <ions> at a common <temperature> in a uniform medium, $\lambda_D^{-2}=\sum_i n_i z_i^2e^2/(\epsilon k_BT)$, where $n_i$ is the bulk <number density> of species $i$.
For a symmetric monovalent electrolyte with density $n$ of each sign, the sum gives $2ne^2/(\epsilon k_BT)$. If the symbol $n$ instead denotes the total <ion> density, the prefactor is $ne^2/(\epsilon k_BT)$. The physics is identical; silently changing the meaning of density creates a spurious factor of $\sqrt2$ in the screening <length>.
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